1. Technical Field of the Invention
The invention relates generally to communication systems; and, more particularly, it relates to coding and decoding of signals employed within such communication systems.
2. Description of Related Art
Data communication systems have been under continual development for many years. One particular type of communication system, turbo code type communication systems, and variants thereof have been the focus of a great deal of interest in the recent years. A primary directive in this area of development has been to try continually to lower the BER (Bit Error Rate) floor for communication channel's having a given SNR (Signal to Noise Ratio. The SNR is oftentimes referred to in terms of the Eb/No (ratio of energy per bit Eb to the Spectral Noise Density No) waterfall part within such a communication system that supports a given BER.
In designing such communication systems and codes employed therein, the ideal goal has been to try reach Shannon's limit in a communication channel. Shannon's limit (sometimes referred to as the communication channel's capacity) may be viewed as being the data rate that is used in a communication channel, having a particular SNR (Signal to Noise Ratio), that will achieve error free transmission through the channel; that is to say, the Shannon's limit is a particular SNR of the communication channel that will support precisely 0.0 BER. In other words, the Shannon limit is the theoretical bound for channel capacity for a given modulation and code rate. The code rate is the ratio of information bits over the total number of bits transmitted within the communication system. In the turbo code context, it is common to refer to code rate (or simply “rate”) of n/m, where n is the number of information bits and m is the total number of bits, and where m>n. The difference between m and n typically is referred to as the number of redundancy bits or parity bits of the encoded signal. Turbo codes typically introduce a degree of redundancy to at least a portion of data prior to transmission through a communication channel. This is oftentimes generally referred to as FEC (Forward Error Correction) coding.
Within the context of turbo code design and other code designs having a common directive to achieve as low of a BER floor as possible, there has been a relatively cohesive agreement within the code design community. There is a common belief that the highest performance codes necessarily reside in the code space of turbo codes employing systematic and linear trellises. In designing such turbo codes, the encoder is typically designed such that the encoder satisfies: (1) Ungerboeck's rule for designing systematic TCM (Trellis Coded Modulation) codes and (2) a minimum Hamming distance or Euclidean distance. In short, there has been a belief that to achieve the best performance within such turbo codes, systematic and linear codes are those codes that will provide the greatest results. However, there has also been a great deal of development in the arena of non-systematic codes. It would be desirable if a non-systematic code could be developed that could provide comparable performance to the systematic and linear codes that currently provide for the greatest performance. Unfortunately, however, the prior art does presently provide for any means by which such a code may be designed or implemented.